1. Field of the Invention
The present invention relates to a photonic switching system to be applied to communications with wide frequency band such as TV, video lecturing and video telephones, and a method for transmitting control signals of said system, in particular, a method for transmitting terminal control signals between a photonic switching system and a subscriber terminal.
2. Description of Related Art
Optical fiber communications engineering has recently made great progress and its scope of application is spreading widely; however, aiming for a more advanced system, studies have been developing in the field of photonic switching systems to switch photonic signals without converting to electric signals.
In photonic switching systems in addition to photonic communications signals, it is necessary to transmit various control signals, such as control and supervisory signals for call connection between photonic switching system and subscriber terminals, control signals being transmitted during the communications and disconnection supervisory signals.
Hitherto, concerning the transmission method of switching connection control signals between photonic switching systems, "Photonic Switching System" recorded in Japanese Patent Laid-open No. 88709/1979 was known to people skilled in this engineering field; however, this system regenerates the switching connection control signals included in part of frequency band of photonic communications signals by converting branched photonic communications signals to electric signals and by filtering them through a low pass filter or a narrow-band pass filter.
FIG. 1 is a partial block diagram showing the main portion of a conventional photonic switching system. A photonic communications signal inputted to a photonic switching system through one of incoming lines 101 to 103 is switched by matrix photonic switch 104 composed of switching elements 115 to 123 and inputted into one of photonic mixers 504-506, for instance, photonic mixer 504 through switch element 123. Subscriber terminal connection control signal 108, in the form of an electric signal to be transmitted from the photonic switching system to subscriber terminal is mixed with the photonic communications signal by photonic mixer 504, after being converted to a photonic signal through light source 503. Output light from photonic mixer 504, is switched by means of matrix photonic switch 111 consisting of switching elements 124 to 132 and transmitted to a subscriber terminal through predetermined outgoing line 112, for example. Thus subscriber terminal connection control signals (hereinafter referred to as "terminal control signals") 108, 109, 110 are transmitted to subscribers together with photonic communication signals via outgoing lines 112, 113, 114.
The outgoing photonic communications signals sent, for example, via outgoing line 112 to, for example, subscriber terminal 220, are subsequently regenerated by demodulator 232 within subscriber terminal 220.
In the conventional transmission method of terminal control signals, expensive light sources 501, 502, 503 are necessary for the photonic switching system. Thus the conventional transmission method of transmitting the terminal control signals has the problem that it requires a light source for the photonic switching system and which consequently increases the cost of the photonic switching system.